While the development of antibiotics has revolutionized medicine, effective treatment of viral infections has proved to be more elusive. In particular, chronic infections, such as those associated with retroviruses and herpesviruses, are considerable health problems, particularly for immunocompromised patients.
The genus of human herpesviruses (HHV) include cytomegalovirus (CMV, also referred to as HHV-5); herpes simplex virus (HSV-1); human herpes virus (HHV-2); herpes varicella-zoster (HHV-3) which occurs clinically as either an acute form known as chickenpox or a chronic form termed shingles; Epstein-Barr virus (HHV4); human B cell lymphotrophic virus (HHV-6); HHV-7, a T cell lymphotrophic virus; and HHV-8, which is associated with Kaposi's Sarcoma.
Infection with herpes viruses leads to a lifelong association of virus and host. Following a primary infection, virus may be shed for a number of years. With CMV and EBV, infection in otherwise healthy individuals is often asymptomatic, and a significant proportion of the adult population harbor these viruses in latent form. In immunocompromised individuals, such as chemotherapy patients, organ transplant patients and in particular AIDS sufferers, latent CMV can be re-activated resulting in microcephaly, hepatosplenomegaly, jaundice, convulsive seizures which may cause mental retardation, mononucleosis, retinitis and even death. In AIDS patients, CMV is a predominant cause of morbidity.
A variety of drugs have been developed to treat herpesvirus infections, including naturally occurring proteins and synthetic nucleoside analogs. For example, the natural antiviral protein, interferon, has been used in the treatment of herpesvirus infections, as have the nucleoside analogs, cytosine-arabinoside, adenine-arabinoside, iodoxyuridine and acyclovir, which is presently the treatment of choice for herpes simplex type I infection. Unfortunately not all of these are effective to treat CMV infection. And, drugs currently used to treat CMV infection, such as ganciclovir (9-(1,3-dihydroxy-2-propoxy)methylguanine) and foscarnet (phosphonoformic acid), have side effects and safety issues.
In vitro selection or SELEX (systematic evolution of ligands by exponential enrichment) procedures (Ellington and Szostak (1990) Nature 346:818-22; Tuerk and Gold (1990) Science 249:505-10) has been used to isolate oligonucleotide molecules with high affinity to a wide variety of low molecular weight targets and large complexes (Yang et al. (1998) P.N.A.S. 95:5462-7; Gold et al. (1995) Ann. Rev. Biochem. 64:763-97.). In these procedures, single-stranded oligonucleotide molecules that exhibited the highest affinity to a target are selected from a pool of randomized sequences by reiterative cycles of selection and amplification.
However, little is known about whether RNA ligands can bind to a human virus and block its infection. Compared to monoclonal and polyclonal antibodies, nucleic acid-based ligands possess similar activity (high affinity and specificity) as well as other unique features (Gold et al. (1995), supra.) However, the nature of the interactions between these ligands and their protein targets might be different from those between antibodies and the same targets. Therefore, targets that are not considered immunogenic to antibodies may be tightly bound by RNA ligands. The development of novel anti-viral reagents is of great interest.
Relevant Literature
The systematic evolution of ligands by exponential enrichment was first described by Tuerk and Gold (1990) Science 249(4968):505-10; and is also described in U.S. Pat. No. 5,496,938, entitled “Methods of Producing Nucleic Acid Ligands”.
Modifications of the SELEX procedure using modified nucleotides are described in U.S. Pat. No. 5,958,691, Pieken et al., Sep. 28, 1999. Such ligands enrich the chemical diversity of the candidate mixture for the SELEX process. Specific examples are provided of nucleic acids containing nucleotides modified at the 2′- and 5-position. Specific 2-OH and 2′-NH2 modified RNA ligands to thrombin are described.